I asked Caroline what she remembered about Thursday June 25th 1987.
“It was my birthday,” she said.
I told her that, of course, I knew that but what about the day itself?
“I’d just finished my first year at university. I probably had a present or two and I must have had a few drinks in the evening. I can’t think of anything else.”
“So, there’s nothing you can remember about the day itself? Nothing memorable?”
“Nothing. Why?”
“Because it was more than just a memorable day, it was a remarkable day and not just for you but for everyone.”
Recently, the Today programme’s feature, “Puzzle for Today” asked:
“In the standard d1d2.m1m2.y1y2y3y4 date format, when is the next date on which every digit will be different?”
After about five minutes, I thought I had the answer to the Today question. That got me wondering when the last one had been.
I found that it had been on Caroline’s birthday, Thursday June 25th 1987 (25.06.1987), which was the last date when every digit written in the dd.mm.yyyy format, was different.
Caroline has had six non-repetitive-digit date birthdays: 1973, 1974, 1978, 1983, 1984 and 1987. She never will have another one. I could never have one because my birthday is 08.02
Think about that for a moment and then think about this: a non-repetitive-digit date won’t happen again until June 17th 2345 (17.06.2345), a gap of 358 years. Neither you, nor I and probably, not even our great-great-great-great-great-great-great grandchildren, will ever experience one.
(If you would like to see why there is such a gap, you may find a “proof” at the end.)
I wanted to see if the unique characteristic of 25.06.1987 was mentioned or celebrated anywhere and so I looked at the Wikipedia entry for “June 25th”.
I was very surprised to find a wide range of obscure facts about June 25th. For example, on the 25th of June 1741, Maria Theresa of Austria was crowned Queen of Hungary and on the 25th of June 1935, the Soviet Union and Colombia established diplomatic relations. However, the interesting, distinctive characteristic of that date in 1987 wasn’t mentioned at all.
I added it: https://en.wikipedia.org/wiki/June_25
Before the 21st century, these non-repetitive-digit dates came around quite often. In March 1947, the month after I was born, there were three of them in four days: March 25th, March 26th and March 28th (25.03.1947, 26.03.1947 and 28.03.1947). Talk about London buses!
There were nine such dates in 1947 and in the whole of the 20th century, there were 360 days that had non-repetitive-digit dates.
There were nine such dates in 1947 and in the whole of the 20th century, there were 360 days that had non-repetitive-digit dates.
The earliest non-repetitive-digit date that I can think of is Tuesday July 26th 1345 (26.07.1345) and although I haven’t the proof, I don’t think an earlier AD date is possible. Please let me know if you can think of one.
How can anyone ever find numbers boring?
Why there won’t be a non-repetitive-digit date for 358 years:
There won’t be any non-repetitive-digit dates in this century, the twenty second or twenty third centuries. There can’t be, because any date from 2000 to 2099 will have y1 = 2 and y2 = 0. Therefore, m1 cannot be 0 and so m1 could only be 1.
But, it can't be because since there are only 12 months, m2 can be only 0, 1 or 2, but any of these values (10, 11, 12) leads to repetitions because of the year digits.
The same reasoning applies to the years 2100 to 2199 and 2200 to 2299 and the first possible date is in 2345.
